acids, bases and neutralization reactions · 124 chapter 4 reactions in aqueous solution! figure...

Reactions in aqueous solutions Acids, Bases and Neutralization reactions

ACIDS

Many acids and bases are industrial and household substances and some are important components of biological fluids. Hydrochloric acid (HCl), for example, is an important industrial chemical and the main constituent of gastric juice in your stomach.

Acids and bases are also common electrolytes.

124 CHAPTER 4 Reactions in Aqueous Solution

! FIGURE 4.5 Vinegar and lemon juiceare common household acids. Ammoniaand baking soda (sodium bicarbonate)are common household bases.

Acetic acid, CH3COOH

Nitric acid, HNO3

Hydrochloric acid, HCl

H

Cl

N

O

C

! FIGURE 4.6 Molecular models ofthree common acids.

equations for precipitation reactions, the anions and cations appear to exchange partners. Thus,we put and together to give CaCO3 and and together to give NaCl. Accord-ing to the solubility guidelines in Table 4.1, CaCO3 is insoluble and NaCl is soluble. The balancedmolecular equation is

In a complete ionic equation, only dissolved strong electrolytes (such as soluble ionic com-pounds) are written as separate ions. As the (aq) designations remind us, CaCl2, Na2CO3, andNaCl are all dissolved in the solution. Furthermore, they are all strong electrolytes. CaCO3 is anionic compound, but it is not soluble. We do not write the formula of any insoluble compoundas its component ions. Thus, the complete ionic equation is

and are spectator ions. Canceling them gives the following net ionic equation:

Check We can check our result by confirming that both the elements and the electric chargeare balanced. Each side has one Ca, one C, and three O, and the net charge on each side equals 0.

Comment If none of the ions in an ionic equation is removed from solution or changed insome way, all ions are spectator ions and a reaction does not occur.

PRACTICE EXERCISEWrite the net ionic equation for the precipitation reaction that occurs when aqueous solutionsof silver nitrate and potassium phosphate are mixed.

Answer:

4.3 | ACIDS, BASES, AND NEUTRALIZATIONREACTIONS

Many acids and bases are industrial and household substances (" FIGURE 4.5), andsome are important components of biological fluids. Hydrochloric acid, for example, isan important industrial chemical and the main constituent of gastric juice in yourstomach. Acids and bases are also common electrolytes.

AcidsAs noted in Section 2.8, acids are substances that ionize in aqueous solution to form hy-drogen ions . Because a hydrogen atom consists of a proton and an electron,is simply a proton. Thus, acids are often called proton donors. Molecular models of threecommon acids are shown in " FIGURE 4.6.

Protons in aqueous solution are solvated by water molecules, just as other cationsare [Figure 4.3(a)]. In writing chemical equations involving protons in water, therefore,we write .

Molecules of different acids ionize to form different numbers of ions. Both HCland HNO3 are monoprotic acids, yielding one per molecule of acid. Sulfuric acid,H2SO4, is a diprotic acid, one that yields two per molecule of acid. The ionization ofH2SO4 and other diprotic acids occurs in two steps:

[4.9]

[4.10]

Although H2SO4 is a strong electrolyte, only the first ionization (Equation 4.9) iscomplete. Thus, aqueous solutions of sulfuric acid contain a mixture of ,

and .The molecule CH3COOH (acetic acid) that we have mentioned frequently is the

primary component in vinegar. Acetic acid has four hydrogens, as Figure 4.6 shows, butonly one of them, the H in the COOH group, is ionized in water. The three other hydro-gens are bound to carbon and do not break their bonds in water.C ¬ H

SO42-1aq2HSO4

-1aq2, H+1aq2HSO4

-1aq2 ∆ H+1aq2 + SO42-1aq2H2SO41aq2 ¡ H+1aq2 + HSO4

-1aq2H+

H+H+

H+1aq2H+H+1aq2

3 Ag+1aq2 + PO43-1aq2 ¡ Ag3PO41s2

Ca2+1aq2 + CO32-1aq2 ¡ CaCO31s2Na+Cl-

CaCO31s2 + 2 Na+1aq2 + 2 Cl-1aq2Ca2+1aq2 + 2 Cl-1aq2 + 2 Na+1aq2 + CO32-1aq2 ¡

CaCl21aq2 + Na2CO31aq2 ¡ CaCO31s2 + 2 NaCl1aq2Cl-Na+CO3

2-Ca2+

Acids are substances that ionize in aqueous solutions to form hydrogen ions H+ (aq). As a hydrogen atom consists of a proton and an electron, H+ is simply a proton. Thus, acids are also called proton donors.

ACIDS

Protons in aqueous solution are solvated by water molecules. In writing chemical equations involving protons in water, therefore, we write H+ (aq).Molecules of different acids ionize to form different numbers of H+ ions. Both HCl and HNO3 are monoprotic acids, yielding one H+ per molecule of acid.

1) H2SO4 (aq) ⟶ H+ (aq) + HSO4- (aq)

HCl




Nitric acid, HNO3


H

Cl

N

O

C








Answer:






[4.9]

[4.10]




SO42-1aq2HSO4

-1aq2, H+1aq2HSO4


-1aq2H+

H+H+

H+1aq2H+H+1aq2

3 Ag+1aq2 + PO43-1aq2 ¡ Ag3PO41s2




2-Ca2+




Nitric acid, HNO3


H

Cl

N

O

C








Answer:






[4.9]

[4.10]




SO42-1aq2HSO4

-1aq2, H+1aq2HSO4


-1aq2H+

H+H+

H+1aq2H+H+1aq2

3 Ag+1aq2 + PO43-1aq2 ¡ Ag3PO41s2




2-Ca2+

HNO3

Sulfuric acid, H2SO4, is a diprotic acid, it yields two H+ per molecule of acid. The ionization of H2SO4 and other diprotic acids occurs in two steps:

2) HSO4- (aq) ⟶ H+ (aq) + SO42- (aq)

ACIDSAcetic acid (CH3COOH) is the primary component in vinegar and is a monoprotic acid.

It has four hydrogens but only one of them, the H in the COOH group, is ionized in water. The three other hydrogens are bound to carbon and do not break their C-H bonds in water.

CH3COOH (aq) ⟶ H+ (aq) + CH3COO- (aq)




Nitric acid, HNO3


H

Cl

N

O

C








Answer:






[4.9]

[4.10]




SO42-1aq2HSO4

-1aq2, H+1aq2HSO4


-1aq2H+

H+H+

H+1aq2H+H+1aq2

3 Ag+1aq2 + PO43-1aq2 ¡ Ag3PO41s2




2-Ca2+

BASES

Bases are substances that accept H+ ions. Bases produce hydroxide ions OH- (aq) when they dissolve in water.

NH3 (aq) + H2O ⟶ NH4+ (aq) + OH- (aq)

Ionic hydroxide compounds, such as NaOH, KOH, and Ca(OH)2, are among the most common bases. When dissolved in water, they release OH- ions into the solution.

Compounds that do not contain OH- ions can also be bases. For example, ammonia (NH3) is a common base. When added to water, it accepts an H+ ion from a water molecule and thereby produces an OH- ion.SECTION 4.3 Acids, Bases, and Neutralization Reactions 125

G I V E I T S O M E T H O U G H TThe structural formula of citric acid, a main component of citrusfruits, is

How many can be generated by each citric acid molecule dissolved in water?

BasesBases are substances that accept (react with) ions. Bases produce hydroxide ions

when they dissolve in water. Ionic hydroxide compounds, such as NaOH, KOH,and Ca(OH)2, are among the most common bases. When dissolved in water, they disso-ciate into ions, introducing ions into the solution.

Compounds that do not contain ions can also be bases. For example, ammo-nia (NH3) is a common base. When added to water, it accepts an ion from a watermolecule and thereby produces an ion (! FIGURE 4.7):

[4.11]

Ammonia is a weak electrolyte because only about 1% of the NH3 forms andions.

Strong and Weak Acids and BasesAcids and bases that are strong electrolytes (completely ionized in solution) are strongacids and strong bases. Those that are weak electrolytes (partly ionized) are weakacids and weak bases. When reactivity depends only on concentration, strongacids are more reactive than weak acids. The reactivity of an acid, however, can dependon the anion as well as on concentration. For example, hydrofluoric acid (HF) isa weak acid (only partly ionized in aqueous solution), but it is very reactive and vigor-ously attacks many substances, including glass. This reactivity is due to the combinedaction of and .

" TABLE 4.2 lists the strong acids and bases we are most likely to encounter. Youneed to commit this information to memory in order to correctly identify strong elec-trolytes and write net ionic equations. The brevity of this list tells us that most acids areweak. (For H2SO4, as we noted earlier, only the first proton completely ionizes.) Theonly common strong bases are the common soluble metal hydroxides. Most other metalhydroxides are insoluble in water. The most common weak base is NH3, which reactswith water to form ions (Equation 4.11).OH-

F-1aq2H+1aq2H+1aq2 H+1aq2

OH-NH4

+NH31aq2 + H2O1l2 ∆ NH4

+1aq2 + OH-1aq2OH-H+

OH-OH-

1OH-2 H+

H+(aq)

C COOH

COOH

COOH

H

HO

H

C

C

H

H H2O OH!NH3

" "

NH4"

# FIGURE 4.7 Hydrogen ion transfer.An H2O molecule acts as a proton donor(acid), and NH3 acts as a proton acceptor(base). Only a fraction of the NH3 moleculesreact with H2O. Consequently, NH3 is aweak electrolyte.

TABLE 4.2 • Common Strong Acids and Bases

Strong Acids Strong Bases

Hydrochloric, HCl Group 1A metal hydroxides [LiOH, NaOH, KOH, RbOH, CsOH]Hydrobromic, HBr Heavy group 2A metal hydroxides [Ca(OH)2, Sr(OH)2, Ba(OH)2]Hydroiodic, HIChloric, HClO3

Perchloric, HClO4

Nitric, HNO3

Sulfuric, H2SO4

Strong and Weak Acids and Bases Acids and bases that are strong electrolytes (completely ionized in solution) are strong acids and strong bases. Those that are weak electrolytes (partly ionized) are weak acids and weak bases.

Common strong acids and basesStrong Acids Strong Bases Hydrochloric, HCl Group 1A metal hydroxides [LiOH, NaOH, KOH, RbOH, CsOH] Hydrobromic, HBr Group 2A metal hydroxides [Ca(OH)2, Sr(OH)2, Ba(OH)2]

Hydroiodic, HIChloric, HClO3

Perchloric, HClO4

Nitric, HNO3

Sulfuric, H2SO4

The brevity of this list tells that most acids are weak. (For H2SO4 only the first proton completely ionizes). The common strong bases are the common soluble metal hydroxides. Most other metal hydroxides are insoluble in water. The most common weak base is NH3, which reacts with water to form OH- ions.

PRACTICE EXERCISE (Comparing Acid Strengths)

• The following diagrams represent aqueous solutions of acids HX, HY, and HZ. Rank the acids from strongest to weakest.

SolutionWe can determine the relative numbers of uncharged molecular species in the diagrams. The strongest acid is the one with the most H+ ions and fewest undissociated molecules in solution. The weakest acid is the one with the largest number of undissociated molecules.

The order is HY > HZ > HX. HY is a strong acid because it is totally ionized (no HY molecules in solution), whereas both HX and HZ are weak acids, whose solutions consist of a mixture of molecules and ions. Because HZ contains more H+ ions and fewer molecules than HX, it is a stronger acid.

• Imagine a diagram showing 10 Na+ ions and 10 OH- ions. If this solution were mixed with the one pictured above for HY, what species would be present in a diagram that represents the combined solutions after any possible reaction?

Answer: The diagram would show10Na+ ions, 2OH-ions, 8Y- ions and 8 H2O molecules.


TABLE 4.3 • Summary of the Electrolytic Behavior of Common Soluble Ionic and Molecular Compounds

Strong Electrolyte Weak Electrolyte Nonelectrolyte

Ionic All None NoneMolecular Strong acids (see Table 4.2) Weak acids, weak bases All other compounds

G I V E I T S O M E T H O U G H TWhy isn’t Al(OH)3 classified as a strong base?

SAMPLE EXERCISE 4.5 Comparing Acid Strengths

The following diagrams represent aqueous solutions of acids HX, HY, and HZ, with watermolecules omitted for clarity. Rank the acids from strongest to weakest.

SOLUTIONAnalyze We are asked to rank three acids from strongest to weakest, based on schematicdrawings of their solutions.

Plan We can determine the relative numbers of uncharged molecular species in the diagrams.The strongest acid is the one with the most ions and fewest undissociated molecules insolution. The weakest acid is the one with the largest number of undissociated molecules.

Solve The order is . HY is a strong acid because it is totally ionized (no HYmolecules in solution), whereas both HX and HZ are weak acids, whose solutions consist of amixture of molecules and ions. Because HZ contains more ions and fewer molecules thanHX, it is a stronger acid.

PRACTICE EXERCISEImagine a diagram showing 10 ions and 10 ions. If this solution were mixed with theone pictured above for HY, what species would be present in a diagram that represents thecombined solutions after any possible reaction?

Answer: The diagram would show 10 ions, 2 ions, 8 ions, and 8 H2O molecules.

Identifying Strong and Weak ElectrolytesIf we remember the common strong acids and bases (Table 4.2) and also remember thatNH3 is a weak base, we can make reasonable predictions about the electrolytic strength ofa great number of water-soluble substances. ! TABLE 4.3 summarizes our observationsabout electrolytes. To classify a soluble substance as strong electrolyte, weak electrolyte,or nonelectrolyte, we work our way down and across this table. We first ask whether thesubstance is ionic or molecular. If it is ionic, it is a strong electrolyte. The second columnof Table 4.3 tells us that all ionic compounds are strong electrolytes. If the substance ismolecular, we ask whether it is an acid or a base. (It is an acid if it either has H first in thechemical formula or contains a COOH group.) If it is an acid, we use Table 4.2 to deter-mine whether it is a strong or weak electrolyte: All strong acids are strong electrolytes,and all weak acids are weak electrolytes. If an acid is not listed in Table 4.2, it is probablya weak acid and therefore a weak electrolyte.

Y-OH-Na+

OH-Na+

H+

HY 7 HZ 7 HX

H+

!

!"

!

!

!!

!!

! !

"

!

!

!

"

"

"

"

""

"

""

"

"

HX HY HZ














Y-OH-Na+

OH-Na+

H+

HY 7 HZ 7 HX

H+

!

!"

!

!

!!

!!

! !

"

!

!

!

"

"

"

"

""

"

""

"

"

HX HY HZ














Y-OH-Na+

OH-Na+

H+

HY 7 HZ 7 HX

H+

!

!"

!

!

!!

!!

! !

"

!

!

!

"

"

"

"

""

"

""

"

"

HX HY HZHX HY HZ

Strong and Weak Acids and Bases

Identifying Strong and Weak Electrolytes

It is possible to make predictions about the electrolytic strength of a great number of water-soluble substances. To classify a soluble substance as strong electrolyte, weak electrolyte, or non-electrolyte, we work our way down and across this table. We first ask whether the substance is ionic or molecular. If it is ionic, it is a strong electrolyte.

Summary of the Electrolytic Behavior of Common Soluble Compounds


Ionic All None None

Molecular Strong acids (see table) Weak acids, weak bases All other compounds

PRACTICE EXERCISE (Identifying Strong, Weak, and Non-electrolytes)

• Classify these dissolved substances as strong, weak, or non-electrolyte: CaCl2, HNO3, C2H5OH (ethanol), HCOOH (formic acid), KOH.

SolutionWe can predict whether a substance is ionic or molecular based on its composition.

Two compounds are ionic: CaCl2 and KOH. Because the “Summary of the Electrolytic Behavior of Common Soluble Compounds“ table tells us that all ionic compounds are strong electrolytes, that is how we classify these two substances. The three remaining compounds are molecular. HNO3 and HCOOH are acids. Nitric acid, HNO3, is a common strong acid (“Common Strong Acids and Bases” table), and therefore is a strong electrolyte. Because most acids are weak acids, the best guess would be that HCOOH is a weak acid (weak electrolyte).

The remaining molecular compound, C2H5OH, is neither an acid nor a base, so it is a non-electrolyte.

Note: Although C2H5OH has an OH group, it is not a metal hydroxide and so not a base. Rather, it is a member of a class of organic compounds that have C-OH bonds, which are known as alcohols.

Identifying Strong and Weak Electrolytes

Neutralization Reactions and Salts The properties of acidic solutions are quite different from those of basic solutions. Acids have a sour taste, whereas bases have a bitter taste.In addition, acidic and basic solutions differ in chemical properties in several other important ways.

When an acid and a base are mixed, a neutralization reaction occurs. The products of the reaction have none of the characteristic properties of either the acidic solution or the basic solution. E.g. When hydrochloric acid is mixed with a solution of sodium hydroxide, the reaction is

HCl (aq) + NaOH (aq) ⟶ H2O (l) + NaCl (aq)acid base water salt

Water and table salt, NaCl, are the products of the reaction. By analogy to this reaction, the term salt has come to mean any ionic compound whose cation comes from a base (for example, Na+ from NaOH) and whose anion comes from an acid (for example, Cl- from HCl). In general, a neutralization reaction between an acid and a metal hydroxide produces water and a salt.

PRACTICE EXERCISE (Writing Chemical Equations for a Neutralization Reaction)

• For the reaction between aqueous solutions of acetic acid (CH3COOH) and barium hydroxide, Ba(OH)2, write the balanced molecular equation

SolutionNeutralization reactions form two products: H2O and a salt. We examine the cation of the base and the anion of the acid to determine the composition of the salt.

The salt contains the cation of the base (Ba2+) and the anion of the acid (CH3COO-). Thus, the salt formula is Ba(CH3COO)2. This compound is soluble in water. The unbalanced molecular equation for the neutralization reaction is

CH3COOH (aq) + Ba(OH)2 (aq) ⟶ H2O (l) + Ba(CH3COO)2 (aq)

To balance this equation, we must provide two molecules of CH COOH to furnish the two CH COO- ions and to supply the two H ions needed to combine with the two OH ions of the base. The balanced molecular equation is

2 CH3COOH (aq) + Ba(OH)2 (aq) ⟶ 2 H2O (l) + Ba(CH3COO)2 (aq)

• For the reaction of phosphorous acid (H3PO3) and potassium hydroxide (KOH), write the balanced molecular equation.

Answers: (a)H3PO3 (aq) + 3 KOH (aq) ⟶ 3 H2O (l) + K3PO3 (aq)

Neutralization Reactions and Salts

acids, bases and neutralization reactions · 124 chapter 4 reactions in aqueous solution! figure...

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